1. Field of the Invention
The present invention relates to a method of manufacturing an oxide superconductor thin film which permits controlling the critical temperature of the superconductor.
2. Description of the Related Art
In order to use an oxide superconductor in various fields, it is necessary to control the critical temperature which is one of the important characteristics of the oxide superconductor. It was customary in the past to control the critical temperature of the oxide superconductor by controlling the ratio of the element substituting each site of the compositional formula of the oxide superconductor. For substituting the element in each site of the compositional formula, it is known to the art to add a raw material containing the substituting element in preparing a raw material mixture. It is also known to the art to dope an oxide shaped body prepared from the raw material mixture with a suitable dopant.
On the other hand, it is reported in "Nature, Vol. 351,549-551, by M. G. Smith et al" that a superconductor was prepared from a bulk material of Sr.sub.1-x Nd.sub.x CuO.sub.2, which is a crystal of tetragonal system. It is reported in this literature that an oxide. superconductor of Sr.sub.1-x Nd.sub.x CuO.sub.2 is manufactured as follows. In the first step, the starting materials of SrCO.sub.3, Nd.sub.2 O.sub.3 and Cu metal, which are mixed at a predetermined mixing ratio to provide a desired chemical composition, are dissolved in nitric acid to dry, thereby obtaining a mixed nitrate. The resultant nitrate is slowly heated to 600.degree. C. to decompose the nitrate. The decomposed nitrate is maintained at 600.degree. C. for one hour. The resultant material is pulverized, mixed and calcined at 600.degree. C. for 3 hours. Then, the calcined material is pulverized and mixed, followed by shaping the resultant powdery material in a desired shape so as to obtain a shaped body. Further, the shaped body is subjected to a heat treatment at 925.degree. C. for 12 to 16 hours, followed by applying an additional heat treatment under a high pressure, i.e., at a temperature of about 1000.degree. C. and a pressure of 25 kbar, to the shaped body for 0.5 hour, quenching the shaped body, and reducing a pressure to ordinary pressure so as to obtain an oxide superconductor of Sr.sub.1-x Nd.sub.x CuO.sub.2.
In order to control the critical temperature of the Sr.sub.1-x Nd.sub.x CuO.sub.2 oxide superconductor, it is also necessary to control the substituting ratio of the element.
However, in the method of controlling the substituting ratio of the element, the maximum mixing amount of the raw material containing the substituting element and the maximum doping amount of the dopant are restricted because the ratio of the substituting element is restricted in order to permit the substituting element to be substituted in the desired site while maintaining the crystal structure of the oxide superconductor. Naturally, the upper limit of the critical temperature is determined by the ratio of the substituting element, making it difficult to obtain an oxide superconductor exhibiting a higher critical temperature. It should also be noted that, in the conventional method exemplified above, it is necessary to apply troublesome treatments such as the treatments under a high total pressure or under a high oxygen partial pressure, leading to a low efficiency of the manufacturing process.